Hitherto, a straight excavating operation such as a horizontal planing operation, a normal planing operation or the like has been performed with a hydraulic excavator such as a power shovel or the like machine by adequately actuating a boom lever, an arm lever and a bucket lever with operator s hand for actuating a boom, an arm and a bucket. However, since the actuations as mentioned above are compositely accomplished, respectively, an operator is required to perform a very troublesome actuating operation with a high skill.
In view of the aforementioned problem, various proposals have been heretofore made with respect to an automation technology employable for a straight excavating operation. FIG. 7 is a graph which illustrates by way of example a conventional automation technology as disclosed in an official gazette of Japanese Published Patent NO. 36135/ 1983. According to the conventional automation technology, a boom 1, an arm 2 and a bucket 3 include turn pins 4, 5 and 6 each of which is equipped with an angle sensor. The angle sensors for the boom 1, the arm 2 and the bucket 3 are practically utilized such that in response to detection signals .theta..sub.1, .theta..sub.2 and .theta..sub.3 from the angle sensors, the y-coordinate of a bucket edge relative to a preset height D preset for a straight excavating operation is calculated with the aid of a computer based on distances l.sub.1, l.sub.2 and l.sub.3 between the turn pins 4, 5 and 6, the preset height D and a distance y.sub.0 from the preset height D up to the turn pin 4 and turnable movement of each of the boom 1, the arm 2 and bucket 3 is then controlled so as to allow the. y-coordinate to be reduced to zero.
However, it has been found that the conventional automation technology has a problem that all the turn pins 4, 5 and 6 for the boom 1, the arm 2 and bucket 3 are required to have an angle sensor attached thereto, respectively. In addition, with respect to the conventional automation technology, since the Y-coordinate of the bucket edge requiring a large quantity of complicated calculating operations is calculated with the aid of the computor, there arises another problem that installation of a computor having a larger capacity is required if a property of responsiveness should be raised up.
The present invention has been made with the foregoing background in mind and its object resides in providing an apparatus for controlling a straight excavating operation with a hydraulic excavator wherein the straight excavating operation can be performed by simple actuations with an excellent property of responsiveness at an inexpensive cost with the aid of the apparatus which is constructed with a simple structure.